Conventional Natural Gas Combined Cycle (“NGCC”) power plants are sometimes co-located with conventional desalination plants since conventional desalination plants consume significant amounts of power. The conventional NGCC plants typically include a large cooling tower to condense the steam produced in the steam cycle. These conventional NGCC plants that are co-located with the conventional desalination plants require additional capital costs and maintenance costs for the salt water cooling tower. These cooling towers typically generate a concentrated seawater blowdown stream, which can be discharged into the seawater. Once-through seawater has been used; but, this process generates thermal pollution which is detrimental to sea life and is typically not environmentally acceptable.
Typical desalination plant membranes, for example, ultrafiltration (“UF”) membranes, nanofiltration (“NF”) membranes, and reverse osmosis (“RO”) membranes, are operable at seawater temperatures up to about 113° F. As the water temperature increases, the RO differential pressure (“DP”) decreases, which results in increasing the membrane capacity. However, the higher water temperature also causes the membranes to have reduced salts rejection. This reduced salt rejection is especially critical for boron since boron is typically the limiting component for desalinated water quality. Boron and chloride removal are especially important for desalinated water that is used for agricultural purposes in arid climates. In arid climates, water evaporation increases and results in increased boron and chloride concentrations within the soil and plants. This boron and chloride build-up reduces crop yields. Thus, the temperature of the seawater should be maintained at appropriate levels to reduce the boron and chloride concentrations in the resulting desalinated water. There is a need in the art for improving the inter-processes between the desalination process and the NGCC process in a cost effective manner when desalination plants are co-located with NGCC plants.
Additionally, United States Gulf Coast (“USGC”) desalination plants are subject to membrane fouling due to biological growth from the warm gulf water, and from elevated silt content produced by the Mississippi River and other high turbidity rivers. Thus, these desalination plants typically require pretreatment and/or acidification of the inlet seawater. Although the example provided below is in conjunction with USGC desalination plants, other desalination plants in other regions of the globe have similar issues.
Silica is typically removed from water and brines using lime softening pretreatment. The silica, magnesium, and calcium are precipitated as a sludge at a pH of about ten to about eleven. Lime or caustic typically is used to increase the pH of the intake seawater. While increasing the pH is effective at removing the silica, essentially all of the magnesium, which is a potentially valuable component in seawater, also is removed. Alternatively, the RO membranes can be used to remove silica; however, this results in membrane scaling and frequent replacement of the RO membranes. Anti-sealants can be used in addition to the RO membranes, but this results in additional costs and also adds additional chemicals in the reject brine concentrate that is sent to disposal.
Acidification of the inlet seawater, which involves lowering the pH to about six or below, is desirable for desalination plants that use seawater that includes silica rich fine silt in a colloidal suspension. Acidification causes some of the particles'surface charges to switch from negative to positive, thereby destabilizing the colloidal suspension and enabling effective settling with conventional flocculants, for example, polymers and/or ferric chloride. Carbonate and bicarbonate removal also is facilitated by the acidification of the inlet seawater. The carbonate and bicarbonate are converted to carbon dioxide (“CO2”) which can be stripped from the seawater using air. Disinfection also is enhanced at the low pHs. Additionally, hypochlorous or hypobromous acid from chlorination remain in their germicidally effective non-dissociated fog in when the pH is acidic. However, at typical seawater pHs of about eight, these acids dissociate and become less effective.
Unfortunately, acidification of the entire desalination feedwater stream to a pH of about six or less is not cost effective since typical desalination plant yields are about forty to about fifty percent. Acidification involves treatment of over two gallons of seawater for every gallon of resulting desalinated water. In addition, the acidic reject brine concentrate would have to be pH neutralized before it could be discharged back to the body of water, such as a gulf, a sea, or an ocean. Any flocculants, disinfectants, and carbonate deficiencies in the reject brine concentrate also have a potentially negative environmental impact.
The low yield of desalinated water increases the cost of the seawater pretreatment systems and reject brine concentrate disposal. For example, silt removal and biological treatment constitute a large portion of the capital cost for USGC desalination plants. For a typical USGC desalination plant, about forty percent of the cost of the desalination plant is attributed to the seawater intake, pretreatment, and concentrate disposal.
The low yield of desalinated water from the RO membranes also requires pressure recovery devices for high efficiency operation. These devices either pressurize the RO feedwater or produce electricity from the high pressure non-permeate stream, which is about 1000 pounds per square inch gage (“psig”). Although these devices allow a high efficiency, they increase the capital cost of the desalination plant and incur additional maintenance costs.
GE Water offers a high efficiency reverse osmosis (“HERO”) process that can achieve about 90-98% recovery of desalinated water. The system uses two pass reverse osmosis membranes operated at a pH of about 10-10.5. The high pH allows greater than about 99.4 percent boron rejection, greater than about 99.97 percent silica rejection, and eliminates bio-fouling. However, this process requires cation pretreatment, or softening, to reduce hardness to less than about 0.1 parts per million (“ppm”), degasification of CO2 to less than about 10 ppm, and caustic addition to raise the pH to above about ten. In addition, the large backwash stream from the cation pretreatment, which includes calcium and magnesium purge, and the reverse osmosis non-permeate stream, which includes sodium, chloride, bromide, and boron purge, require disposal. The high cost of pretreatment and concentrated waste streams typically make this process uneconomical for seawater desalination. There is a need in the art for improving the pretreatment process for seawater intake streams that are used in desalination plants.
There are some regions, for example the Middle East, India, California, and Florida, in the world that have issues with water availability. The Middle East is one of the most arid regions in the world and it has been predicted that regional per capita water availability will fall by about half by 2050, with serious consequences for the region's already stressed aquifers and natural hydrological systems. Currently, the Middle East has about sixty percent of the world's 7500 desalination plants, which produces about half of the entire region's drinking water. These desalination plants use the available seawater to produce desalinated water that may be used for drinking.
Most of these regional countries in the Middle East have coastlines that provide easy access to unlimited supplies of seawater. Desalination plants currently use membranes, such as an ultrafiltration (“UF”), a nanofiltration (“NF”), and a reverse osmosis (“RO”), and thermal separation units, such as a multistage flash (“MSF”) and a mechanical vapor recompression (“MVR”), to extract desalinated water from seawater. The hot residue brine, which is an environmentally harmful saline liquid, is then returned to the sea. In addition, once through cooling water is used for overall heat rejection from the thermal processes and also is returned to the sea. These discharges that are sent back to the sea result in increased salinity and temperature in a large region around the desalination outfall. The increased salinity and temperature create environmental issues and limits the amount of desalination capacity that can be installed within a given region. These issues are especially true for the Red Sea and the Arabian Gulf where there is rapid growth in desalination capacity. The limited circulation, lack of fresh water inflow, and large natural evaporation rate due to the increased water temperatures create elevated salinity, about 4.5% versus 3.5% total dissolved solids (“TDS”), in these seas.
The brines discharged back into the sea impact marine ecosystems. Desalination plants along the Arabian Gulf treat in excess of 25 million cubic meters per day of seawater to produce up to 10 million cubic meters per day of drinking water, with the difference being returned to the sea as hot and concentrated brine. There is currently no binding legislation in the region that mandates quality control on effluents and no technology is currently available to treat these massive discharges.
Concerns have arisen about the long-term impact of desalination in the waters of the Arabian Gulf by using current desalination technology. Water can only be desalinated two to five times and it takes up to 200 years for the whole Arabian Gulf to be renewed by natural currents from the Indian Ocean.
In view of the foregoing discussion, need is apparent in the art for reducing and/or eliminating the brine stream being recycled into the sea. Additionally, a need is apparent for increasing the efficiency of producing desalinated water from seawater. Also, there exists the need for recovering the minerals during the desalination process in an economic and reusable manner. Further, there exists a need for utilizing any by-products formed during the desalination process. Furthermore, there is a need in the art for improving the inter-processes between the desalination process and the NGCC process when desalination plants are co-located with NGCC plants. Moreover, there is a need in the art for improving the pretreatment process for seawater intake streams that are used in desalination plants. A technology addressing one or more such needs, or some other related shortcoming in the field, would benefit desalination processes. One or more of these technologies are included within the current embodiments of the invention.
The drawings illustrate only exemplary embodiments of the invention and are therefore not to be considered limiting of its scope, as the invention may admit to other equally effective embodiments.